Strain sensing optical cable with low vibration attenuation construction

ABSTRACT

A strain sensing optical fiber cable is provided. The cable includes at least one optical fiber embedded in the cable jacket such that vibrations from the environment are transmitted into the cable jacket to the optical fiber. The cable is configured in a variety of ways, including through spatial arrangement of the sensing fibers, through acoustic impedance matched materials, through internal vibration reflecting structures, and/or through acoustic lens features to enhance sensitivity of the cable for strain/vibration detection/monitoring.

PRIORITY APPLICATION

This application claims the benefit of U.S. Provisional Application No.62/513,011, filed on May 31, 2017, the content of which is relied uponand incorporated herein by reference in its entirety.

BACKGROUND

The disclosure relates generally to a strain sensing fiber-optical cableconfigured for strain sensing and more particularly to a fiber opticcable configured for enhanced sensitivity to strain and/or vibrationsensing. Strain within an optical fiber can be measured by measuring thechange in a transmission property of a signal along the optical fiber(e.g., Rayleigh scattering of an optical signal carried along thefiber). Vibrations in an environment in contact with an optical fibercable cause dynamic strain within the optical fibers of the cable, whichin turn can be monitored/detected by measuring/detecting the straininduced scattering, for example measuring/detecting the strain-inducedchanges in the amplitude and/or phase of the scattered signal. Somevibration detection systems are configured to detect specific strainevents and are able to indicate where along the length of the cable thestrain event occurs. In addition, systems may be able to detect/monitora dynamic or static strain signature, strain magnitude, and strainduration of the event. Typical systems have along the length detectionchannels about every 5-10 m. So for example a 1 km long system wouldhave 200-100 detection channels.

SUMMARY

One embodiment of the disclosure relates to a strain sensing cable fordetecting vibrations in an environment. The cable includes a cablejacket, a first optical fiber embedded within the cable jacket, a secondoptical fiber embedded within the cable jacket and a tensile strengthelement embedded in the cable jacket. When viewed in cross-section takenperpendicular to a longitudinal axis of the cable jacket, the cablejacket defines a first axis and a second axis. The first axis intersectsthe first optical fiber, the second optical fiber and the tensilestrength element, and the tensile strength element is located betweenthe first optical fiber and the second optical fiber along the firstaxis. The second axis is perpendicular to the first axis and is locatedat the midpoint between the first and second optical fibers. The firstand second optical fibers each have a length within plus or minus 0.5%of a length of the tensile strength element such that both the first andsecond optical fibers experience strain caused by vibrations transmittedinto the cable jacket from the environment.

An additional embodiment of the disclosure relates to a strain sensingcable. The cable includes a jacket defining an outer surface, a firstoptical fiber embedded within the jacket such that the first opticalfiber experiences strain applied to the cable and a second optical fiberembedded within the jacket such that the second optical fiberexperiences strain applied to the cable. The cable includes a tensilestrength element embedded in the jacket, and the tensile strengthelement is located between the first optical fiber and the secondoptical fiber along a first axis of the jacket. When viewed incross-section taken perpendicular to a longitudinal axis of the jacket,the first optical fiber has an unobstructed field of view to the outersurface of the jacket that has an uninterrupted arc angle greater than180 degrees, and the second optical fiber has an unobstructed field ofview to the outer surface of the jacket that has an uninterrupted arcangle greater than 180 degrees.

An additional embodiment of the disclosure relates to a strain sensingcable. The cable includes a jacket defining a central horizontal axis, acentral vertical axis and an outermost surface of the cable. The cableincludes a first tensile strength element embedded in the jacket locatedon a first side of the vertical axis and a second tensile strengthelement embedded in the jacket located on a second side of the verticalaxis opposite the first side. The cable includes a first optical fiberembedded within the jacket and located on the first side of the verticalaxis such that the first tensile strength element is located between thefirst optical fiber and the vertical axis. The cable includes a secondoptical fiber embedded within the jacket and located on the second sideof the vertical axis such that the second tensile strength element islocated between the second optical fiber and the vertical axis.

Additional features and advantages will be set forth in the detaileddescription which follows, and in part will be readily apparent to thoseskilled in the art from the description or recognized by practicing theembodiments as described in the written description and claims hereof,as well as the appended drawings.

It is to be understood that both the foregoing general description andthe following detailed description are merely exemplary, and areintended to provide an overview or framework to understand the natureand character of the claims.

The accompanying drawings are included to provide a furtherunderstanding and are incorporated in and constitute a part of thisspecification. The drawings illustrate one or more embodiment(s), andtogether with the description serve to explain principles and operationof the various embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic view of a system for monitoring/detectingvibration utilizing a fiber optic cable according to an exemplaryembodiment.

FIG. 2 shows a longitudinal cross-sectional view of a vibration sensingfiber optic cable, according to an exemplary embodiment.

FIG. 3 shows a longitudinal cross-sectional view of a vibration sensingfiber optic cable, according to another exemplary embodiment.

FIG. 4 shows a longitudinal cross-sectional view of a vibration sensingfiber optic cable, according to another exemplary embodiment.

FIG. 5 shows a longitudinal cross-sectional view of a vibration sensingfiber optic cable, according to another exemplary embodiment.

FIG. 6 shows a longitudinal cross-sectional view of a vibration sensingfiber optic cable, according to another exemplary embodiment.

FIG. 7 shows a longitudinal cross-sectional view of a vibration sensingfiber optic cable, according to another exemplary embodiment.

FIG. 8 shows a longitudinal cross-sectional view of a vibration sensingfiber optic cable, according to another exemplary embodiment.

FIG. 9 shows a longitudinal cross-sectional view of a vibration sensingfiber optic cable, according to another exemplary embodiment.

FIG. 10 is a plot showing the relationship of acoustic impedance of animpedance matching material to vibration power transmission through acable jacket, according to an exemplary embodiment.

FIG. 11 shows a plot of the relationships of impedance and speed ofsound vs. modulus of elasticity assuming TPU density of 1.2 g/cm³according to an exemplary embodiment.

FIG. 12 shows a longitudinal cross-sectional view of a vibration sensingfiber optic cable, according to another exemplary embodiment.

FIG. 13 shows a detailed view of an acoustic reflector of the cable ofFIG. 12, according to an exemplary embodiment.

FIG. 14 shows a longitudinal cross-sectional view of a vibration sensingfiber optic cable, according to another exemplary embodiment.

FIG. 15 shows a longitudinal cross-sectional view of a vibration sensingfiber optic cable, according to another exemplary embodiment.

FIG. 16 shows a longitudinal cross-sectional view of a vibration sensingfiber optic cable, according to another exemplary embodiment.

FIGS. 17A and 17B show a schematic diagram and a plot of cable radiusvs. angle of refractive energy transfer for a vibration sensing cablemodel that takes into account the radius of curvature and dimensions ofthe cable jacket and fiber and how they apply to calculate a maximumpotential angle for direct energy transfer.

FIG. 18 shows a longitudinal cross-sectional view of a vibration sensingfiber optic cable, according to another exemplary embodiment.

DETAILED DESCRIPTION

Referring generally to the figures, various embodiments of an opticalfiber cable configured for sensing strain is shown and described. Inspecific embodiments, the optical fiber cables discussed herein areconfigured to detect vibration applied to the cable body. Vibrationexperienced by optical fibers within a cable cause scattering of opticalsignals within the optical fiber which can be detected/monitored suchthat the optical cable can function as a vibration sensor/detector. Suchvibration sensors may be useful in variety of applications includingperimeter monitoring applications, pipeline monitoring applications,etc.

For example, in a perimeter-monitoring application, the vibrationsensing optical fiber cable is buried in the ground. Vibration withinthe ground (e.g., from a vehicle, person, etc. moving near or over theperimeter) is transmitted into the cable causing change in thescattering of the optical signal within the optical fibers of the cable.Detection electronics are connected to the cable to detect/monitor thescattering of the optical signal and to provide an indication/alarm whenvibration indicative of perimeter breach are detected. The optical cableembodiments discussed herein can be utilized in other vibrationmonitoring applications, including pipeline line leak/breakagemonitoring. In various embodiments, the sensing systems discussed hereinmay utilize a variety of sensing modalities, includingmonitoring/sensing changes in Rayleigh, Raman and/or Brilliounscattering. In some embodiments, the optical cable embodiments discussedherein are used in distributed acoustic sensing (DAS) systems ordistributed strain sensing (DSS) systems, and in other embodiments, theoptical cable embodiments discussed herein used in distributedtemperature sensing systems, In some embodiments, the optical cableembodiments discussed herein combine vibration monitoring applications(e.g., DAS or DSS applications) and temperature monitoring applications(e.g., DTS applications). As will generally be understood DTSapplications utilize measurement of Raman scattering for temperaturedetection.

As discussed in more detail herein, in order to improve vibrationdetection, Applicant has to develop a number of optical fiber cabledesigns configured to improve vibration transmission into the cablejacket and into the optical fiber. By increasing the transmission ofvibration from the environment (e.g., the ground, monitored pipeline,etc.) into the cable jacket and ultimately into the optical fiber,Applicant believes that the cable design discussed herein increases thesensitivity of the cable for vibration detection.

As will generally be understood regarding vibration transmission, theamount of vibrational energy transmitted across an interface between twomaterials is inversely related to the difference in acoustic impedancesof the two materials. In general, the percentage of vibrational powerreflected from an interface, R, is governed by the following Equation 1:

$R = \left( \frac{\left( {Z_{2} - Z_{1}} \right)}{\left( {Z_{2} + Z_{1}} \right)} \right)^{2}$

where Z₁ and Z₂ are the acoustic impedances of the two materials thatform the interface. Thus, the more similar the acoustic impedances ofmaterials forming the interface, the greater the percentage ofvibrational energy transmitted across the interface and the lower thepercentage of vibrational energy reflected off of the interface.Similarly, the fewer material interfaces that a vibration/sound waveneeds to traverse, the greater the vibration transmission will be.

In specific embodiments, the optical fiber cable embodiments discussedherein are configured to improve/facilitate vibration transmission fromthe environment and to the sensing optical fibers of the cable. Forexample, in various embodiments (as shown for example in FIGS. 2-8),cables discussed herein are designed to position the sensing opticalfibers within the cable jacket and relative to other cable components(e.g., strength members, armor layers, etc.) in a manner that increasesvibration transmission to the sensing optical fibers. For example, invarious embodiments, the sensing fiber(s) of the cable embodimentsdiscussed herein are located adjacent to the surface of the cable andpositioned to minimize the extent to which other cable components, suchas strength elements, block or shield the sensing optical fibers fromincoming vibrations. In such embodiments, the sensing optical fiber(s)are positioned external to the strength elements such that the opticalfibers have a large, unobstructed radial field of view to the outersurface of the optical fiber cable. In addition, in some embodiments, apair of optical fibers are located on either side of the strengthmember(s) such that the cable provides sensing fibers that haveunobstructed views of vibrations entering the cable jacket from eitherlateral side of the cable and from above and below the cable.

In addition to (or instead of) positioning of the sensing optical fibersrelative to the cable jacket and cable components, in variousembodiments, an impedance matching material is utilized between thevibration-containing environment and the vibration sensing cable toincrease sensitivity for vibration detection. In general, Applicantbelieves that use of an impedance matching material as discussed hereindecreases the proportion of vibrational power reflected off interfacesbetween the environment and the sensing optical fiber. In someembodiments, the impedance matching material is an outer layer of thecable jacket, and in other embodiments, the impedance matching materialis a material added to the environment adjacent the vibration sensingcable.

Further, in various embodiments, sensing optical fiber cables discussedherein utilize one or more aspect of cable design to focus vibrationalenergy onto the sensing optical fiber(s). In some embodiments, vibrationsensing cables discussed herein include one or more acoustic reflectorsembedded in the cable jacket that reflect vibrations toward the opticalfiber. In yet additional embodiments, the cable jacket may have an outersurface shaped to direct vibrations toward the sensing optical fiber viarefraction as the vibration is transmitted into the cable jacket. In yetadditional embodiments, the outer radius of curvature of the cablejacket may be sized relative to the radius fiber to increase the amountof vibrational energy that is directed toward the sensing optical fibervia refraction as the vibration is transmitted into the cable jacket.

Referring to FIG. 1, a system 10 for detecting vibration in anenvironment, such as ground 12, is shown according to an exemplaryembodiment. In general, system 10 includes vibration sensing electronics14 and a strain or vibration sensing cable, shown as optical fiber cable16. In general, cable 16 includes one or more vibration sensing opticalfibers, and sensing electronics 14 are configured to detect changes inscattering of the optical signal carried within the optical fiber(s) ofcable 16 indicative of vibration within ground 12. Vibration sensingelectronics are coupled to the cable 16 and are configured to determinean aspect of vibration in the environment (e.g., vibration occurrence,vibration magnitude, vibration duration, vibration direction, etc.)based on optical scattering of an optical signal within the sensingoptical fiber(s) of cable 16 that is caused by the vibrationstransmitted into cable 16.

For example, vibrations detected in ground 12 may include vibrations 18traveling upward (e.g., vibrations reflected off of bedrock) and/orvibrations 20 traveling horizontally from a vibration source. Sensingelectronics 14 may be configured to log, store, process, provide alerts,etc. in response to detected vibrations that are indicative of an eventthat system 10 is configured to monitor. For example, in a perimetermonitoring application, system 10 may be configured to detect vibrationsassociated with movement (e.g., people, vehicles, etc.) crossing orapproaching cable 16. In other embodiments, the monitored environmentmay be a pipeline or other conduit, and system 10 is configured todetect vibrations associated with a leak or break in the pipeline. Itshould also be understood that cable 16 of system 10 may be any one ofthe cable embodiments discussed herein. Similarly, in some embodiments,cable 16 may be an optical fiber cable including any combination ofcable features, and specifically any of the vibration detectionenhancement features of any of the cable embodiments discussed herein.

Referring to FIG. 2, a cross-sectional view of a strain (e.g., dynamicstrain) or vibration sensing optical fiber cable 30 is shown accordingto an exemplary embodiment. Cable 30 includes a cable jacket, outerjacket or sheath, shown as jacket 32. In specific embodiments, jacket 32is formed from one or more layer of an extruded material (e.g., anextruded polymer material) that supports the other components of cable30. In the embodiment shown, jacket 32 is the outer layer of cable 30and forms outermost surface 34 of cable 30. In this arrangement, whenviewed in the longitudinal cross-section of FIG. 2, outer surface 34 isa contiguous surface that surrounds the various internal components(e.g., sensing fibers, strength elements, etc. as discussed below) ofcable 30.

As can be seen in FIG. 1, when cable 30 is located within the desiredenvironment (e.g., within ground 12), outer surface 34 is the surface ofcable 30 that engages or interfaces with the environment carrying thevibrations to be detected/monitored using cable 30. The material ofjacket 32 may be any material used in cable manufacturing, such aspolyethylene, polyvinyl chloride (PVC), polyvinylidene difluoride(PVDF), nylon, polyester or polycarbonate and their copolymers,polyurethane and specifically thermoplastic polyurethane. In a specificembodiment, the material of cable jacket 32 may be a material that hasan acoustic impedance selected that is similar/the same as the acousticimpedance the environment. For example, in one embodiment where thevibration-carrying environment is the ground, jacket 32 may be formedfrom a material, such as medium density polyethylene, having an acousticimpedance less than 2 MRayl, specifically between 1 and 2 MRayl, andeven more specifically between 1.5 and 2 MRayl. In some otherembodiments, jacket 32 is formed from a TPU material having an acousticimpedance of between 0.8 and 1 MRayl and specifically of 0.85 to 0.95MRayl.

Cable 30 includes at least one strain sensing or vibration sensingoptical fiber, shown as first sensing optical fiber 36 and secondsensing optical fiber 38, coupled to jacket 32. As shown in FIG. 2,sensing optical fibers 36 and 38 is located within jacket 32 and,specifically, are embedded within the material of jacket 32. In thisembodiment, outer surfaces of sensing optical fibers 36 and 38 are incontact with and are coupled to the material of jacket 32 such thatvibrations experienced by cable 30 are transmitted effectively tosensing optical fibers 36 and 38. In specific embodiments, the outersurfaces of sensing optical fibers are defined by an outer polymercoating 52 (e.g., a UV cured acrylate coating) that surrounds a glasscore and cladding layers, shown generally together at 50, in FIG. 2.

In some embodiments, to facilitate transmission of vibrational energyfrom the environment to sensing fibers 36 and 38, sensing fibers 36 and38 may have a low level of excess fiber length (EFL). Low levels of EFLhelp ensure that fiber strain-inducing vibrations are efficientlytransmitted from the environment, to cable jacket 32 and to sensingfibers 36 and 38. In specific embodiments, EFL of sensing fibers 36 and38 can be expressed in relation to the longitudinal length of cablejacket 32 and/or to the longitudinal length of strength elements 40 and42. In specific embodiments, sensing fibers 36 and 38 each have alongitudinal length within plus or minus 0.5%, specifically plus orminus 0.1%, more specifically plus or minus 0.05% and even morespecifically, plus or minus 0.01% of the longitudinal length of strengthmember 40, strength member 42 and/or jacket 32.

In various embodiments, cable 30 includes one or more elongate tensilestrength element, shown as strength members 40 and 42. In general,strength members 40 and 42 act to provide structural and tensile supportto cable 30. In the embodiment shown, strength members 40 and 42 areelongate, generally cylindrical or rod-like members embedded within thematerial of jacket 32. In these embodiments, strength members 40 and 42have outer surfaces that are coupled to the material of jacket 32 suchthat the material of jacket 32 is in contact with the outer surfaces ofstrength members 40 and 42. Strength members 40 and 42 may generally beformed from a rigid material, more rigid than the material of cablejacket 32, and in various embodiments, strength members 40 and 42 may bemetal, braided steel, glass-reinforced plastic, fiber glass, fiber glassyarns or other suitable material.

While strength members 40 and 42 provide strength to cable 30, thestrong materials that form strength members 40 and 42 typically havehigh acoustic impedances, and thus, will tend to reflect a significantportion of vibrational energy that is transmitted on a path to sensingfibers 36 and 38 that intersects strength members 40 and/or 42. To limitthe vibration blocking that may otherwise be caused by strength members40 and 42, cable 30 is structured in a variety of ways in order tofacilitate exposure of sensing fibers 36 and 38 to the vibrations thatcable 30 receives from the environment. For example, cable jacket 32,sensing fibers 36 and 38 and strength elements 40 and 42 may be shapedand/or arranged in a manner that Applicant believes will increase theamount of vibrational energy transmitted to sensing fibers 36 and 38.For example, as shown in FIG. 2, sensing fibers 36 and 38 are locatedadjacent lateral or minor outer surfaces of cable jacket 32 with bothstrength members 40 and 42 located toward the midpoint of cable jacket32. As will be explained in more detail below, Applicant believes thatthis positioning allows sensing fibers 36 and 38 to be more directlyexposed to vibrations from the environment while limiting the vibrationblocking effect of strength members 40 and 42.

FIG. 2 shows a cross-sectional view taken perpendicular to the length orlongitudinal axis of cable 30. In this view, cable 30 defines a firstaxis, shown as horizontal axis 44, and a second axis, shown as verticalaxis 46. As shown, horizontal axis 44 and vertical axis 46 areperpendicular to each other and intersect at the central position of thelongitudinal axis. As shown in FIG. 2, cable 30 is arranged such thathorizontal axis 44 intersects sensing fibers 36 and 38 and intersectsstrength members 40 and 42 with strength members 40 and 42 being locatedbetween sensing fibers 36 and 38 along horizontal axis 44. In thespecific embodiment shown in FIG. 2, horizontal axis 44 intersects thelongitudinal center points of sensing fibers 36 and 38 and of strengthmembers 40 and 42.

Further, sensing fiber 36 is located on the opposite side of verticalaxis 46 from sensing fiber 38, and strength member 40 is located on theopposite side of vertical axis 46 from strength member 42. Applicantbelieves that by positioning sensing fibers 36 and 38 along axis 44(e.g., which is typically referred to as the cable's neutral axis)limits the amount of tensile and compressive strain that sensing fibers36 and 38 experience during normal bending and handling of cable 30. Bylimiting these unwanted sources of strain, the optical scatteringassociated with bending and handling is reduced, which, in turn,improves the sensitivity (e.g., decreases the signal to noise ratio) ofcable 30 to vibration-induced strain.

Further, sensing fibers 36 and 38 may be positioned close to outersurface 34. In various embodiments, sensing fibers 36 and 38 are locatedadjacent to outer surface 34 of cable jacket 32 such that a minimumdistance, shown as D1, from the outer surface of sensing fibers 36 and38 to the outer surface 34 is less than or equal to 0.5 mm. Further, inthe cable arrangement/shape of the embodiment of FIG. 2, D1 for bothsensing fibers 36 and 38 reside along horizontal axis 44. However, inother embodiments, jacket 32 may have other shapes and/or sensing fibers36 and 38 may be positioned such that the minimum distance representedby D1 does not lie along horizontal axis 44.

In such embodiments, Applicant believes that by positioning sensingfibers 36 and 38 close to outer surface 34 (and further away fromstrength elements 40 and 42) vibration reception may be enhanced bydecreasing the vibration blockage/reflection that may otherwise becaused by strength elements 40 and 42. Similarly, by positioning sensingfibers 36 and 38 near outer surface 34, the degree to which the materialof jacket 32 attenuates the vibrations traveling through jacket 32 tosensing fibers 36 and 38 is decreased (as compared to sensing fibersburied deeper with the material of a cable jacket).

Further, cable 30 may be shaped in a manner to further facilitate use ofcable 30 in a vibration sensing application. For example, as shown inFIG. 2, an outer dimension of cable jacket 32 taken along horizontalaxis 44 is greater than an outer dimension of cable jacket 32 takenalong vertical axis 46. In a specific embodiment, cable jacket 32 has anoblong shape such that the outer dimension of cable jacket 32 takenalong horizontal axis 44 is at least twice the outer dimension of cablejacket 32 taken along vertical axis 46. In a specific embodiment, themaximum outer dimension of cable jacket 32 in the direction of verticalaxis 46 is less than or equal to 2 mm, which as discussed belowregarding FIGS. 17A and 17B, is believed to provide for improvedrefractory channeling of acoustic waves toward sensing fibers 36 and 38.This shape facilitates the outward, shallow positioning of sensingfibers 36 and 38. The preferential bend characteristics may alsofacilitate horizontal positioning (e.g., burying) of cable 30 within theground via a tool such as a vibratory plow.

Still referring to FIG. 2, the shape of cable jacket 32 and thepositioning of sensing fibers 36 and 38 relative to strength members 40and 42 allows sensing fibers 36 and 38 to have a high level of directexposure to vibrations transmitted into cable jacket 32. As shown inFIG. 2, this high level exposure is shown as the unobstructed field ofview that each sensing fiber 36 and 38 has to a portion of outer surface34 of cable 30. As used herein, the field of view of sensing fiber 36and 38 relates to the portion of outer surface 34, measured in terms ofthe labeled arc angle, that has only the material of jacket 32 locatedbetween the outer surface 34 and sensing fiber 36 and 38 (e.g., withoutany intervening blocking structures). This parameter provides anindication of the extent to which vibrations are permitted to traveldirectly through jacket 32 to sensing fibers 36 and 38 without othercable structures, such as strength elements 40 and 42,blocking/reflecting vibrations before they reach sensing fibers 36 and38.

As shown in FIG. 2, sensing fiber 36 has an unobstructed field of viewof outer surface 34 represented by arc 54, and sensing fiber 38 has anunobstructed field of view of outer surface 34 represented by arc 56. Asshown, the unobstructed field of views 54 and 56 of sensing fibers 36and 38 are uninterrupted in that they provide a continuous view of outersurface 34 along the entire arc angle shown. In various embodiments,unobstructed field of views 54 and 56 have arc angles greater than 180degrees, specifically are between 180 degrees and 300 degrees and morespecifically are between 180 degrees and 270 degrees. As shown in FIG.2, unobstructed views 54 and 56 face in opposite directions from eachother and both are symmetric vertically about horizontal axis 44. As canbe seen in FIG. 2, the positioning of sensing fibers 36 and 38 providesfor unblocked vibration reception from a large portion of potentialvibration transmission directions around the perimeter of cable 30.

Further referring to FIG. 2, cable 30 may also include one or moreadditional optical fibers 60. Optical fibers 60 are located toward thecenter of cable jacket 32 along horizontal axis 44. In this arrangement,horizontal axis 44 intersects additional optical fibers 60 andadditional optical fibers 60 are located between sensing fibers 36 and38 and between strength elements 40 and 42 along horizontal axis 44.

In specific embodiments, additional optical fibers 60 may be additionalvibration/strain sensing optical fibers embedded within jacket 32 andhaving a low EFL as discussed above regarding sensing fibers 36 and 38.In such embodiments, additional sensing optical fibers 60 may provideunobstructed fields of view to the upper and lower central portions 58of outer surface 34 for which sensing fibers 36 and 38 do not haveunobstructed views. In addition, by providing additional sensing fibers60 at different spatial positioning along axis 44, cable 30 allowssystem 10 to be configured to determine various additionalcharacteristics of vibration within the ground, such as directionalityof the vibration waves, based on the differential response of sensingfibers 36, 38 and 60 when exposed to a particular vibration. In otherembodiments, the one or more additional optical fibers 60 may betelecommunications optical fibers. In various embodiments, additionalfibers 60 may be in the form of an optical fiber ribbon supported bycable jacket 32.

In addition to the various features discussed above to improvesensitivity to vibration, aspects of cable 30 may also facilitate use,deployment and handling of cable 30. For example, in one embodiment, theoblong shape and strength element positioning of cable 30 results in afiber with a preferential bend characteristic such that cable 30 tendsto bend in the direction of vertical axis 46. This preferential bendcharacteristic may facilitate deployment of cable 30 in the horizontalposition (e.g., as shown in FIG. 2) via burying equipment such asburying via vibratory plow equipment. In addition, cable jacket 32 mayhave co-extruded tear features 62 that facilitate access to fibers 36,38 and 60 as may be needed for splicing to other optical fiber cablesand/or for connection to sensing electronics 14.

Referring to FIG. 3, a strain or vibration sensing optical fiber cable70 is shown according to an exemplary embodiment. Cable 70 issubstantially the same as cable 30, except for the differences discussedherein. Cable 70 includes one or more additional optical fibers, shownas a third optical fiber 72, located within a buffer tube 74. Thirdoptical fiber 72 is positioned between sensing fibers 36 and 38 alongthe horizontal axis 44, and generally is located at the center of cable70 in both the vertical and horizontal positions.

In specific embodiments, third optical fiber 72 is a strain-isolatedoptical fiber that has a greater level of EFL than fibers 36 and 38 suchthat third optical fiber 72 does not experience strain/vibrationsexperienced by cable 70. In some such embodiments, third optical fiber72 has a longitudinal length that is at least 0.01% greater. In suchembodiments, third optical fiber 72 acts as a temperature-sensing fiberallowing system 10 to account for the effect that temperature has on theRaman scattering of optical signals transmitted on fibers 36 and 38. Inthis arrangement, third optical fiber 72 provides a stress-free opticalfiber that provides a baseline level scattering that is utilized bysystem 10 to improve the accuracy of vibration/strain detection based onthe optical scattering measured on sensing fibers 36 and 38. In someembodiments, the temperature reading from third optical fiber 72 may beutilized to provide DTS functionality to cable 70.

Referring to FIG. 4, a strain or vibration sensing optical fiber cable80 is shown according to an exemplary embodiment. Cable 80 issubstantially the same as cable 30, except for the differences discussedherein. Cable 80 is configured to further increase the unobstructedfields of view 54 and 56 by extending the spacing along horizontal axis44 between sensing fibers 36 and 38 and strength members 40 and 42,respectively.

Cable 80 includes a pair of ribs, shown as ribs 82 and 84 that extendoutward from cable jacket 32 in the direction along horizontal axis 44.Sensing fiber 36 is embedded in the material of rib 82, and sensingfiber 38 is embedded in the material of rib 84. In general, ribs 82 and84 are protruding ribs that extend the entire longitudinal length ofcable 80. In the specific embodiment shown, ribs 82 and 84 are formedfrom the same material and are integral and continuous with the materialof cable jacket 32. In specific embodiments, ribs 82 and 84 are formedduring extrusion of the jacket polymer material along with the rest ofcable jacket 32. In a specific embodiment, ribs 82 and/or 84 areremovable (e.g., via tearing) from the remainder of cable jacket 32. Theability to remove ribs 82 and 84 may also allow coiling of fibers 36 and38 for local access or an improved point location for acoustic signal.

As can be seen in FIG. 4, by shifting the positioning of sensing fibers36 and 38 further away from strength elements 40 and 42, respectively,the arc angles of the unobstructed field of views 54 and 56 of sensingfibers 36 and 38 can be increased. In the embodiment shown in FIG. 4,the arc angles of the unobstructed field of views 54 and 56 are greaterthan 270 degrees, specifically are between 270 degrees and 320 degrees,and more specifically are between 290 degrees and 310 degrees.

In addition, as shown in FIG. 4, ribs 82 and 84 have a relatively smallcross-sectional width and height, compared to the diameter of sensingfibers 36 and 38, which allows sensing fibers 36 and 38 to reside closethe outer surfaces of ribs 82 and 84. Thus, by embedding sensing fibers36 and 38 within ribs 82 and 84, a larger portion (compared to cable 30)of the circumference of sensing fibers 36 and 38 is separated from theenvironment only by a thin layer of cable jacket material. In a specificembodiment, more than 180 degrees of the circumference of sensing fibers36 and 38 is less than 0.5 mm from the outer surfaces of ribs 82 and 84.By decreasing the amount of jacket material that vibrations from a widedegree of angles must travel through before reaching sensing fibers 36and 38, the vibration attenuation that occurs within the jacket materialis decreased, which in turn is believed to increase the sensitivity ofcable 80 to vibration detection.

Referring to FIG. 5, a strain or vibration sensing optical fiber cable90 is shown according to an exemplary embodiment. Cable 90 issubstantially the same as cable 80, except for the differences discussedherein. Cable 90 includes one or more strain isolated optical fiber 72located within tube 74 as discussed above regarding cable 70.

Referring to FIG. 6, a strain or vibration sensing optical fiber cable100 is shown according to an exemplary embodiment. Cable 100 issubstantially the same as cable 30, except for the differences discussedherein. Cable 100 includes rectangular shaped cable jacket 102.

Cable 100 includes a single, centrally located strength element 104embedded in cable jacket 102. Strength element 104 is a non-roundstrength element that is positioned within cable jacket 102 such thatthe major axis of strength element 104 is generally aligned with thehorizontal axis 44 of cable 100. In the specific embodiment shown,strength element 104 has a generally rectangular cross-sectional shape.

As shown in FIG. 6, strength element 104 is located between sensingfibers 36 and 38 such that the major axis of strength element 104 isaligned with sensing fibers 36 and 38. In this arrangement, sensingfibers 36 and 38 are embedded in cable jacket 102 such that sensingfibers 36 and 38 are located between the shorter sides 106 ofrectangular strength element 104 and the shorter sides 108 ofrectangular cable jacket 102. In such embodiments, the flat, rectangularshape of cable 100 may facilitate placement of cable 100 in the groundwith axis 44 aligned horizontally (e.g., perpendicular to gravity).Applicant believes that such horizontal positioning may improvesensitivity of cable 100 to sound/vibration waves (e.g., see vibrations18 and 20 in FIG. 1) by ensuring an orientation of cable 100 in theground that positions sensing fibers 36 and 38 to receivesound/vibration waves traveling horizontally or vertically within theground.

Referring to FIG. 7, a strain or vibration sensing optical fiber cable110 is shown according to an exemplary embodiment. Cable 110 issubstantially the same as cable 100, except for the differencesdiscussed herein. Cable 110 includes two additional sensing fibers,shown as sensing fibers 114 and 116. Sensing fibers 114 and 116 arelocated on opposite sides of strength element 112 along the verticalaxis of cable 110. In this arrangement, sensing fibers 114 and 116 arelocated between the long sides 118 of rectangular jacket 102 and thelong sides 120 of strength element 112. Sensing fibers 114 and 116 arepositioned to provide unobstructed fields of view to the upper and lower(in the orientation of FIG. 7) surfaces of cable jacket 102. Thus,sensing fibers 114 and 116 when combined with sensing fibers 36 and 38provide unobstructed fields of view to the entire perimeter/outersurface of cable 110.

In specific embodiments, strength element 112 is shaped to facilitateplacement of sensing fibers 114 and 116 as shown in FIG. 7. In thisembodiment, strength element 112 is shaped to include a pair ofdepressions, cut-outs or channels 122 located along the long sides 120of strength element 112. Sensing fibers 114 and 116 are each located atleast partially within one of the channels 122. This allows for cable110 to support the additional sensing fibers 114 and 116 as shown inFIG. 7, without increasing the vertical outer dimension of cable jacket102.

In addition, placement of sensing fibers 114 and 116 within channels 122brings sensing fibers 114 and 116 closer to the neutral bending axis(shown as the horizontal axis 44 in FIG. 7) which in turn limits theamount of bending-based strain experienced by fibers 114 and 116.However, in some embodiments, because sensing fibers 114 and 116 doexperience some bending-based strain, the strain-based scattering onfibers 114 and 116 can be measured/analyzed to determine the shape ofcable 110 when deployed. As will be understood, positive strain on oneof fibers 114 or 116 indicates that the fiber is on the outside of abend and a negative strain on one of fibers 114 or 116 indicates thatthe fiber is on the inside of a bend. This strain information can beused to determine the position of and degree of bends along the lengthcable 110.

Referring to FIG. 8, a strain or vibration sensing optical fiber cable130 is shown according to an exemplary embodiment. Cable 130 issubstantially the same as cable 30, except for the differences discussedherein. Cable 130 includes a generally cylindrical cable jacket 132 thatdefines an outer surface 134 having a generally circular cross-sectionalshape. In this embodiment, the outer surface 134 is radially symmetricabout the cable's longitudinal axis, while sensing fibers 36 and 38 andstrength elements 40 and 42 are generally aligned along a common axis,shown as horizontal axis 44 in the orientation of FIG. 8. In thisarrangement, even though outer surface 134 is radially symmetric, thearrangement of strength elements 40 and 42, as shown in FIG. 8, createsa preferential bend axis and sensing fibers 36 and 38 are located alongthe neutral axis, which limits their exposure to bending-related strain,as discussed above.

Referring to FIG. 9, a strain or vibration sensing optical fiber cable200 is shown according to an exemplary embodiment. Cable 200 issubstantially the same as cable 30, except for the differences discussedherein. Cable 200 includes a cable jacket 202 that like cable jacket 32surrounds and protects sensing fibers 36 and 38, strength members 40 and42 and additional optical fiber(s) 60. However, cable 200 utilizes animpedance matching material, shown as outer cable jacket layer 204, andan inner cable jacket, shown as inner layer 206. In the embodiment shownin FIG. 9, sensing fibers 36 and 38 are embedded in the inner layer 206such that layer 206 provides protection to fibers 36 and 38 frommechanical damage.

Outer layer 204 at least partially surrounds inner layer 206 (e.g., whenviewed in longitudinal cross-section as shown in FIG. 9). In thespecific embodiment shown in FIG. 9, outer layer 204 defines the outermost surface 34 of cable jacket 202 and completely surrounds inner layer206. Outer layer 204 and inner layer 206 are each continuous, contiguouslayers of material that extend the length of cable 200 (e.g., the entirelength between the opposing first and second ends of the cable).

In general, outer layer 204 is formed from a material that provides foracoustic impedance matching between the material of the environment inwhich cable 200 is installed (e.g., ground 12 as shown in FIG. 1) andthe material of inner layer 206. In such embodiments, the environment(e.g., ground 12 as shown in FIG. 1) has an acoustic impedance, Z1,outer layer 204 is formed from a first material that has an acousticimpedance, Z2, and inner layer 206 is formed from a second material thathas an acoustic impedance, Z3.

In various embodiments, such as a buried cable, perimeter monitoringapplication shown in FIG. 1, Z2 is less than Z3. In such embodiments,outer layer 204 has an acoustic impedance Z2 that is greater than Z1 andis less than Z3. As can be seen through an application of Equation 1 andas will be discussed in more detail below regarding FIG. 10 and Table 1,interposing the impedance matching material of outer layer 204 betweenthe environment and inner layer 206 decreases the amount of vibrationalpower reflected at the material interfaces between the environment andsensing fibers 36 and 38. This decrease in reflected vibrational powerprovided by the acoustic impedance matching of outer layer 204translates into a significant increase in the proportion of vibrationalenergy that is allowed to reach sensing fibers 36 and 38.

As will generally be understood, the acoustic impedance Z2 of thematerial of outer layer 204 typically is selected based on a balancebetween the matching acoustic impedance of the environment and matchingthe acoustic impedance of inner layer 206. This balance related to theacoustic impedance of an impedance matching material in a buried cableapplication is depicted in FIG. 10. In specific embodiments, Z2 is lessthan 85% of Z3, specifically is between 10% and 80% of Z3 and morespecifically is between 20% and 70% of Z3. However, in some embodimentswhere the environment has a high acoustic impedance (e.g., a metalconduit or other structure), Z2 may be greater than Z3.

In specific embodiments, outer layer 204 is formed from a first polymermaterial, and inner layer 206 is formed from a second polymer material.In general, the first polymer material has a different acousticimpedance than the second polymer material, and in applications for usein environments where Z1 is less than Z3, Z2 is less than Z3 and greaterthan Z1 in order to decrease the impedance mismatch from the environmentto the cable jacket. In specific embodiments, Z2 is between 0.3 and 2MRayl, and Z3 is between 1 and 2.5 MRayl. In other specific embodiments,Z2 is between 0.4 and 1.4 MRayl, and Z3 is between 1.5 and 2 MRayl. Inanother specific embodiment, Z2 is between 0.8 and 2 MRayl, andspecifically is 0.9 MRayl. In some embodiments, cable 200 with theseacoustic impedances ranges is intended for use in a ground-basedvibration detection system, where the ground typically has an acousticimpedance between 0.1 MRayl and 0.3 MRayl. In various embodiments, innerlayer 206 is an olefin material, such as low density polyethylene mediumdensity polyethylene, a high density polyethylene, polypropylene, etc.,and outer layer 204 is at least one of a silicone rubber material, anethylene vinyl acetate material and a polyurethane material.

Further as will generally be understood, specific acoustic impedance isa function of the modulus of elasticity of the material, E, and thedensity of the material, ρ, as shown by the following equation, Equation2: Z=(ρE)^(1/2). In various embodiments, outer layer 204 is formed froma material having a density, ρ₁, and a Young's modulus of elasticity,E₁, and inner layer 206 is formed from a material that has a density,ρ₂, and a Young's modulus of elasticity, E₂. Thus, in specificembodiments, the materials of outer layer 204 and inner layer 206 areselected such that (ρ₁E₁)^(1/2) is less than (ρ₂E₂)^(1/2).

In specific embodiments, outer layer 204 is formed from a polymermaterial that has a Young's modulus of elasticity between 150 and 700MPa, specifically 160 MPa to 650 MPa, and more specifically of 165 MPaor 640 MPa, and inner layer 206 is formed from a polymer material thathas a Young's modulus of elasticity between 300 and 1000 MPa. In suchembodiments, outer layer 204 is formed from a polymer material that hasa density between 1.1 and 1.3 g/cm³, and inner layer 206 is formed froma polymer material that has a density between 0.91 and 0.97 g/cm³. In aspecific embodiment, outer layer 204 is a thermoplastic polyurethaneelastomer material having a density of 1.2 g/cm³ and a Young's modulusof elasticity between 200 and 500 MPa.

Still referring to FIG. 9, in various embodiments, the relativethicknesses of layers 204 and 206 are selected to limit attenuation ofvibrations during transmission through jacket 202. Limiting thisattenuation within outer layer 204 may be particularly important when alow modulus material, but higher attenuation material, is used for outerlayer 204. Thus as shown in FIG. 9, outer layer 204 has an averagethickness, represented by T1, and inner layer has an average radialdimension, represented by R1. The average thickness T1 is the averagethickness of outer layer 204 around the entire perimeter of inner layer206, and the average radial dimension R1, is the average radialdimension of inner layer 206 around the entire perimeter of inner layer206. In various embodiments, T1 is less than 30% of R1, specifically isless than 15% of R1 and more specifically is less than 10% of R1. Invarious embodiments, T1 is 10% to 30% of R1, and specifically T1 is 10%to 15% of R1.

Referring to FIG. 9 and FIG. 1, system 10 may utilize an impedancematching material to improve the sensitivity of cable 16 to vibrationswithin the environment. In such embodiments, cable 16 is in contact withthe environment, specifically ground 12, such that vibrations in theground are transmitted into cable 16. In various embodiments, system 10includes an impedance matching material located between ground 12 andthe outer surface of cable 16. In such embodiments, the impedancematching material is in contact with ground 12 and with the cable jacketof cable 16 such that vibrations within ground 12 are transmitted fromground 12 into the impedance matching material then into the cablejacket of cable 16. The vibrations are then transmitted through thecable jacket to the sensing optical fiber of cable 16 (e.g., sensingfibers 36 and 38) which in turn causes optical scattering which isdetected by vibration sensing electronics 14, as discussed above.

In embodiments of system 10 that utilize cable 200, this impedancematching material is outer layer 204, as discussed above. However, inother embodiments, the impedance matching material may be a separatecomponent or material positioned between the environment and cable 16 toprovide the impedance matching functionality discussed above regardingouter layer 204. In embodiments where the acoustic impedance of theenvironment (e.g., ground 12) is less than the acoustic impedance of thematerial of the cable jacket of cable 16, the acoustic impedance of theimpedance matching material is greater than the acoustic impedance ofthe environment and less than the acoustic impedance of the cablejacket.

For example in some such embodiments, the separate impedance matchingmaterial may be an oil material (e.g., mineral oil), a gel materialand/or a polymer material (e.g., SAP polymer material) that is added toground 12 in order to raise the impedance of the area of the groundimmediately adjacent cable 16. In specific embodiments, cable 16 is atleast partially buried within ground 12 and the separate impedancematching material is added to ground within the trench or channeladjacent cable 16. In some embodiments, a separate impedance matchingmaterial may be used in combination with cable 200 to provide two layersof impedance matching material.

In various embodiments, the sensitivity of system 10 utilizing avibration sensing cable, such as cables 30, 200, etc., can be evaluatedin terms of vibrational power transfer across the cable jacket to thesensing optical fiber(s) 36 and/or 38. In specific embodiments, thecable jacket of cable 16 is configured such that at least 25% of thepower of vibrations in the environment, such as ground 12, that areincident on the outer surface of the cable jacket of cable 16 istransmitted through the cable jacket to at least one of sensing opticalfibers 36 and 38. In a more specific embodiment, the cable jacket ofcable 16 is configured such that at least 50% of the power of vibrationsin the environment, such as ground 12, that are incident on the outersurface of the cable jacket of cable 16 is transmitted through the cablejacket to at least one of sensing optical fibers 36 and 38. In specificembodiments, power transfer proportions are calculated based on theenvironment and cable jacket materials utilizing equation 1 above, andin other embodiments, power transfer proportions are determined viatesting.

In a specific embodiment, the cable jacket of cable 16 is configuredsuch that at least 25% of the power of vibrations in the environment,such as ground 12, that are incident on the outer surface of the cablejacket of cable 16 is transmitted through the cable jacket to at leastone of sensing optical fibers 36 and 38, when the acoustic impedance,Z1, of ground 12 is 0.1 MRayl. In another specific embodiment, the cablejacket of cable 16 is configured such that at least 50% of the power ofvibrations in the environment, such as ground 12, that are incident onthe outer surface of the cable jacket of cable 16 is transmitted throughthe cable jacket to at least one of sensing optical fibers 36 and 38,when the acoustic impedance, Z1, of ground 12 is 0.3 MRayl.

As will be understood, in order to provide a desired level of impedancematching, the acoustic impedance of the impedance matching material,whether in the form of outer layer 204 of cable 200 or a separateimpedance matching material added to the environment, will be selectedto be relatively close to the acoustic impedance of the environmentcarrying the vibrations. In specific embodiments, the acoustic impedanceof the impedance matching material, Z2, is within 2 MRayl, specificallywithin 1.1 MRayl and more specifically within 0.4 MRayl, of the acousticimpedance of the environment, Z1. In a specific embodiment where theenvironment is ground 12, ground 12 may have an acoustic impedance of0.1 to 0.3 MRayl.

Referring to FIGS. 10 and 11 and Table 1 below, modeling data foracoustic transmission from the ground through a variety of cable jacketshaving different material properties and layer configurations are shownand described.

TABLE 1 Mat 1 Z1 Mat 2 Z2 P_(t)12 Mat 3 Z3 P_(t)23 P_(t)13 Ground 0.3HDPE 2.3 41% 0.1 2.3 16% Ground 0.3 MDPE 1.8 49% 0.1 1.8 20% Ground 0.3MDPE 1.8 49% MDPE 1.8 100.0% 49% 0.1 1.8 20% MDPE 1.8 100.0% 20% Ground0.3 Dow Silastic 1.16 65% MDPE 1.8 95.3% 62% 0.1 Rubber 1.16 29% MDPE1.8 95.3% 28% GP45, 45 Durometer Ground 0.3 Impedance 0.73 83% MDPE 1.882.1% 68% 0.1 Matched 0.43 61% MDPE 1.8 62.3% 38% Material

Table 1 shows modeling data for vibration power transmission from theground through cable jackets having one layer of either HDPE or MDPE(top six rows). In addition, Table 1 shows modeling data for vibrationpower transmission from the ground through cable jackets having outerlayers 204 of either Silastic Rubber GP45 from Dow Chemical or acalculated Impedance Matched Material and an inner layer 206 of MDPE.

As can be seen in Table 1, both Silastic Rubber GP45 and the ImpedanceMatched Material increase vibration power transmission percentage (shownin column P_(t)13) compared to the single layer MDPE or HDPE jacketmaterials (shown in column P_(t)12). Specifically, Table 1 shows theestimated power transmission gains through use of the calculatedacoustic Impedance Matched Material is between about 22% and 27%relative to a typical HDPE cable jacket and between about 8-13% for useSilastic Rubber GP45 material relative to a typical HDPE cable jacket.

FIG. 10 is a plot showing the effect of the acoustic impedance, Z2, ofthe material of outer layer 204, on vibration power transmissionutilizing a inner layer 206 of MDPE having a Z3 of 1.8 MRayl for soilhaving a Z1 of both 0.1 and 0.3. As will be understood, the shape of thecurves in FIG. 10 illustrate the balance between matching theenvironment's acoustic impedance and the cable jacket's acousticimpedance when selecting an acoustic impedance matching material. Inaddition, from these plots the 0.43 MRayl and 0.73 MRayl values for Z2of the calculated Impedance Matched Material, shown in Table 1, aredetermined. In such embodiments, the TPU material having the acousticimpedance 0.43 MRayl has a modulus of elasticity of 155 MPa, and the TPUmaterial having the acoustic impedance 0.73 MRayl has a modulus ofelasticity of 640 MPa,

FIG. 11 shows a plot of the relationships of impedance and speed ofsound vs. modulus of elasticity assuming TPU density of 1.2 g/cm³. Thus,FIG. 11 shows that for a TPU density of 1.2 g/cm³, a TPU materials(specifically Irogran A80P with a specific gravity of 1.09, availablefrom Huntsman) a speed of sound based calculated modulus of elasticityin the range of 155 MPa and 640 MPa would result in acoustic impedancesof 0.43 to 0.73 MRayl, respectively.

Referring to FIG. 12, a strain or vibration sensing optical fiber cable300 is shown according to an exemplary embodiment. Cable 300 issubstantially the same as cable 30, except for the differences discussedherein. Cable 300 includes one or more acoustic reflector, shown asreflectors 302 and 304, embedded in cable jacket 32. In general,reflectors 302 and 304 are formed from a material that has an acousticimpedance greater than the acoustic impedance of the material of cablejacket 32. Reflector 302 has a vibration-reflecting surface, shown asconcave surface 306, and reflector 304 has a vibration-reflectingsurface, shown as concave surface 308. As shown in FIG. 12, reflectors302 and 304 are embedded in the material of cable jacket 32 such thatthe material of cable jacket 32 is in contact with surfaces 306 and 308,and specifically, cable jacket 32 may surround reflectors 302 and 304such the reflectors are completely embedded within cable jacket 32.

Referring to FIG. 13, a detailed view of reflector 304 is shownillustrating vibrational reflection provided by reflector 304. It shouldbe understood that reflector 302 functions the same as reflector 304.Vibrational waves, shown schematically as horizontal vibrations 310, aretransmitted through a portion of outer surface 34 into cable jacket 32.The horizontal vibrations 310 continue to travel through cable jacket 32until they encounter reflector 304. As illustrated by Equation 1 above,a large portion of the power of vibrations 310 reflect off of reflector304 due to the acoustic impedance difference between the material ofreflector 304 and of the material of jacket 32. The concave shape ofsurface 308 reflects portions of vibrations 310 (that would haveotherwise missed sensing fiber 38) toward sensing fiber 38.

As will be understood from Equation 1, the greater the acousticimpedance difference between the material of reflectors 302 and 304 andof cable jacket 32, the larger the proportion of vibrational power thatis reflected back toward sensing fibers 36 and 38. In variousembodiments, the acoustic impedance of the material of reflectors 302and 304 is at least twice, specifically is at lease 5× and morespecifically is at least 10× of the acoustic impedance of the materialof cable jacket 32.

A wide variety of materials may be used to form reflectors 302 and 304.In some embodiments, reflectors 302 and 304 may be formed from a highacoustic impedance polymer material, and in such embodiments reflectors302 and 304 may be coextruded with jacket 32. In other embodiments,reflectors 302 and 304 may be separate structures around which jacket 32is extruded. In exemplary embodiments, reflectors 302 and 304 may be ametal material or a high density polymer material, and in a specificembodiment, reflectors 302 and 304 may be formed from an aluminum Mylarmaterial.

As shown in FIG. 12, sensing fibers 36 and 38 are located alonghorizontal axis 44 and are positioned adjacent opposite ends of cablejacket 32 with strength member 40 in between the two sensing fibers. Inthis arrangement, reflector 302 is located between sensing fiber 36 andtensile strength member 40 along horizontal axis 44, and reflector 304is located between sensing fiber 38 and tensile strength member 40 alonghorizontal axis 44. In this arrangement, reflecting surfaces 306 and 308face in opposite directions from each other along horizontal axis 44. Inthe particular embodiment shown in FIG. 12, strength element 40 is acentrally located strength element that is coaxial with the longitudinalaxis of cable 300 and is equidistant from reflectors 302 and 304 andfrom sensing fibers 36 and 38. In other embodiments, two strengthelements and/or additional optical fibers may be located alonghorizontal axis 44 between reflectors 302 and 304.

As shown in FIG. 12, reflectors 302 and 304 are positioned and shapeddifferent from other materials/layers that may be found in typical fiberoptic cables. For example, unlike typical buffer tubes or wrapped armorlayers, concave surfaces 306 and 308 of reflectors 302 and 304 are incontact with the material of cable jacket 32. In addition, concavesurfaces 306 and 308 of reflectors 302 and 304 define arc angles lessthan 360 degrees (i.e., they do not circumscribe fibers 36 and 38),specifically less than 270 degrees, and more specifically less than 180degrees.

In specific embodiments, cable 300 has a width dimension (in thehorizontal direction in the orientation of FIG. 12) between 3 mm and 4mm, specifically of 3.5 mm, and a height dimension (in the verticaldirection in the orientation of FIG. 12) between 1 mm and 2 mm,specifically of 1.8 mm. In such embodiments, strength member 40 has anouter diameter of 1 mm. In such embodiments, the height of reflectors302 and 304 is between 0.5 mm and 2 mm and specifically is 1 mm. In suchembodiments, sensing fibers 36 and 38 are located a distance of between0.1 mm and 0.5 mm, and specifically 0.25 mm from surfaces 306 and 308,respectively, along horizontal axis 44.

Referring to FIG. 14, a strain or vibration sensing optical fiber cable320 is shown according to an exemplary embodiment. Cable 320 issubstantially the same as cable 300, except for the differencesdiscussed herein. Cable 320 includes a single sensing optical fiber 36generally located along the central longitudinal axis of cable 320.

Cable 320 includes a first pair of acoustic reflectors, 322 and 324, anda second pair of acoustic reflectors, 326 and 328. Like the reflectorsof cable 300, acoustic reflectors 322, 324, 326 and 328 are formed froma material having an acoustic impedance greater than the acousticimpedance of the material of cable jacket 32 and each has a concavevibration reflecting surface 329, 330, 332, 334, respectively.

The reflecting surfaces 329, 330, 332, 334, are shaped and positionedsuch that incoming vibration waves, shown schematically at 310, arereflected off of reflecting surfaces 329, 330, 332, 334 and directedtoward sensing fiber 36. Specifically, concave reflecting surfaces 329,330, 332, 334 each face sensing fiber 36 and are concave relative tosensing fiber 36. In other embodiments, acoustic reflectors 322, 324,326 and 328 may have a variety of other shapes including bead shaped orrectangular shapes.

In the embodiment shown in FIG. 14, reflectors 322, 324, 326 and 328 arecoupled to strength elements 40 and 42. In specific embodiments,reflectors 322, 324, 326 and 328 may be integral (e.g., coextruded,molded, etc.) with strength members 40, and in other embodiments,reflectors 322, 324, 326 and 328 may separate components embedded injacket 32 adjacent to and contacting strength members 40 and 42. In someembodiments, strength members 40 and 42 and reflectors 322, 324, 326 and328 are all formed from the same high acoustic impedance material aseach other, and in another embodiment, reflectors 322, 324, 326 and 328are formed from a material that is different from and has a higheracoustic impedance than the material of strength members 40 and 42.

As shown, each of strength elements 40 and 42 have a convex outersurface 336, and reflectors 322, 324, 326 and 328 are each coupled tothe convex outer surface 336 of one of strength members 40 and 42. Insuch embodiments, reflectors 322, 324, 326 and 328 may provide bothacoustic reflecting and additional strength to cable 320. In theparticular arrangement shown, reflectors 322 and 324 are located onopposite sides of horizontal axis 44 from each other, and specificallyare spaced 180 degrees from each other around strength element 40.Similarly, reflectors 326 and 328 are located on opposite sides ofhorizontal axis 44 from each other, and specifically are spaced 180degrees from each other around strength element 42.

Referring to FIG. 15, a strain or vibration sensing optical fiber cable340 is shown according to an exemplary embodiment. Cable 340 issubstantially the same as cable 320, except for the differencesdiscussed herein. Cable 340 has a cylindrically shaped cable jacket 342that defines a cylindrical outer surface 344. Cable 340 has a centrallylocated strength member 40.

Cable 340 includes a plurality of acoustic reflectors, shown asreflectors 346, 348, 350 and 352 coupled to and surrounding strengthmember 40. Reflectors 346, 348, 350 and 352 may be integral (e.g.,coextruded, molded, etc.) with strength members 40, and in otherembodiments, reflectors 346, 348, 350 and 352 may separate componentsembedded in jacket 342 adjacent to strength member 40. In someembodiments, strength member 40 and reflectors 346, 348, 350 and 352 areall formed from the same high acoustic impedance material as each other,and in another embodiment, reflectors 346, 348, 350 and 352 are formedfrom a material that is different from and has a higher acousticimpedance than the material of strength member 40.

As shown in FIG. 15, cable 340 includes sensing fibers 36 and 38 spacedfrom each other along horizontal axis 44 and a pair of additionalsensing fibers 356 and 358 that are spaced from each other along thevertical axis of cable 340. Reflectors 346, 348, 350 and 352 haveconcave acoustic reflecting surfaces 360, 362, 364 and 366. Reflectingsurfaces 360, 362, 364 and 366 each face and are concave relative to anassociated sensing fiber such that incoming vibrations are reflectedtoward the associated sensing fiber.

In this arrangement, cable 340 includes four sensing fibers each spacedapproximately 90 degrees from each other. In this arrangement cable 340is radially symmetric. In this arrangement, cable 340 is configured todetect vibrations received from 360 degrees around the perimeter ofcable 340 with one sensing fiber in each quadrant of the cable, whilemaintaining a small, compact form factor.

In a specific embodiment, cable 340 has an outer diameter between 1.5 mmand 3 mm, and specifically of 2 mm. In such embodiments, sensing fibers36, 38, 356 and 358 have outer diameters of 250 microns. In suchembodiments, the radial distance from the center point of strengthmember 40 to the center point of each sensing fibers 36, 38, 356 and 358is between 0.2 mm and 1.3 mm and more specifically is 0.6 mm. In aspecific embodiment, strength element 40 is a steel strength elementhaving an outer diameter of 0.7 mm. Table 2 below shows the relationbetween the fiber offset positioning, bend radius and fiber strain fordifferent arrangements of cable 340.

TABLE 2 Max Additional Fiber Center Fiber Strain Offset Distance (mm)Bend Radius (in) 0.3 0.6 0.9 1.2 2 0.84% 1.43% 2.02% 2.61% 3 0.56% 0.95%1.35% 1.74% 4 0.42% 0.71% 1.01% 1.30% 5 0.33% 0.57% 0.81% 1.04% 6 0.28%0.48% 0.67% 0.87% 7 0.24% 0.41% 0.58% 0.75% 8 0.21% 0.36% 0.50% 0.65% 90.19% 0.32% 0.45% 0.58% 10 0.17% 0.29% 0.40% 0.52% 15 0.11% 0.19% 0.27%0.35% 20 0.08% 0.14% 0.20% 0.26%

Referring to FIG. 16, a strain or vibration sensing optical fiber cable380 is shown according to an exemplary embodiment. Cable 380 issubstantially the same as cable 30, except for the differences discussedherein. Cable 380 has a cable jacket 382, and a single optical sensingfiber 36. In this embodiment, sensing fiber 36 is a tight-bufferedoptical fiber having a tight buffer layer 384 coupled to and surroundingsensing fiber 36.

Cable 380 includes an outer surface 388 that surrounds sensing fiber 36.In the particular embodiment shown, cable jacket 382 defines a part orall of outer surface 388. In contrast to typical cable arrangements,outer surface 388 includes a concave portion 390 that is concaverelative to the exterior of cable 380.

In general, concave surface 390 is shaped and positioned relative tosensing fiber 36 such that incoming vibrations, representedschematically at 310, are refracted as the incoming vibrations aretransmitted into cable jacket 382. As shown schematically in FIG. 16, asvibrations 310 enter cable jacket 382 they are refracted to a newdirection of travel or path 392. Concave surface 390 is shaped and/orpositioned relative to sensing fiber 36 in a manner that increases theproportion of vibrations 310 that are directed toward sensing fiber 36.In particular, concave surface 390 is positioned such that is symmetricabout an axis, shown as horizontal axis 44 that intersects sensing fiber36.

As will be understood, the appropriate radius of curvature for theconcave surface 390 will be determined based on the speed of sound inthe jacket material used. The exact shape of the cable transitionbetween region of planar only sensitivity to combined planar/reflectedenergy can be optimized depending on the strength of the planar wave vs.reflected wave. It is believed that the transition would be a functionof depth of bedrock and distance from the vibration producing event todetermine the ratio of planar wave energy to reflected wave energy.

Referring to FIGS. 17A, 17B and 18, in various embodiments, the radiusesof cable jackets (e.g., the radius of cylindrical cable jackets, theradiuses at the corners of oblong cable jackets, etc.) may be selectedin relation to sensing fiber size and positioning to increase theproportion of vibrational energy directed toward the sensing fiber viarefraction as the vibrations are transmitted into the cable jacketmaterial. Referring to FIG. 17A and FIG. 17B, a model that takes intoaccount the radius of curvature and dimensions of the cable jacket andof the fiber and how they apply to calculate a maximum potential anglefor direct energy transfer are shown.

Based on this model, a limiting angle is determined based on thepotential angular profile and the critical angle for reflectance thatincrease the proportion of vibrational energy directed toward thesensing fiber via refraction. Since the speed of sound in the cablejacket is higher than the surrounding soil, FIG. 17B shows that acousticwaves will be refracted out away from the center of the cable. Thus, tocounter the tendency to refract acoustic waves away from the sensingfiber at the center of the cable, the curvature of the cable can beselected to increase the proportion of acoustic waves refracted towardthe sensing fiber.

Referring to FIG. 17A and 17B, the ability to transfer increased ormaximum acoustic energy to a sensing fiber, such as fibers 36 and 38,from the soil is a function of both the cable materials and componentdimensions. Assuming the sensing fiber is centered inside the radius ofcurvature of the outer surface of the jacket, the component dimensionsfor the sensing fiber and cable radius determine the potential angle fordirect energy transfer (2θ_(cf)) from the acoustic wave (see FIG. 17A).The critical angle for refractive energy transfer (Snell's Law) θ_(c1)is a function of material properties between the soil (or otherenvironment) and the cable jacket. A limiting angle for energy transferis determined by the limiting (smallest) value between the dimensionalcable properties and the critical angle. This is observed in FIG. 17Bwhere the speed of sound for soil is assumed to be 250 m/s. For thiscase, transfer of vibrations from soil to a polyethylene cable jacket,is limited to a maximum angular surface for energy transfer of 14.7degrees due to the critical angle between the two materials. Above acable radius of about 1 mm for 250 micron optical fiber and a cableradius of about 3.5 mm for 900 micron tight buffered optical fiber, themaximum angular surface for energy transfer is limited by the potentialangle 2θ_(cf).

Incorporating a material with a lower speed of sound will increase theeffective region of acoustic energy transfer (see cable 200 in FIG. 9).For example, Dow Silastic Rubber (shown in Table 1 above) is limited toa maximum angular surface for energy transfer of 28.4 degrees(significantly greater than polyethylene). To obtain the benefit for 250micron fiber, the cable radius should be below 1 mm, preferably at ˜0.5mm. To obtain the benefit for 900 micron tight buffered fiber, the cableradius should be below 3.5 mm, preferably at or below ˜1.8 mm.

Thus, based on this analysis, relative sizing/positioning for the cablejacket radius and fiber diameter and positioning can be determined toincrease transfer of vibrational energy to sensing fibers. In anexemplary embodiment, FIG. 18 shows a vibration sensing cable 400 thatutilizes these concepts. Vibration sensing cable 400 is substantiallythe same as cable 200 except as discussed herein. Vibration sensingcable 400 is arranged based on the modeling shown in FIGS. 17A and 17Bto increase refraction-based vibration direction to sensing fibers 36and 38.

As shown in FIG. 18, sensing fibers 36 and 38 each include a tightbuffered layer 402. In this embodiment where sensing fibers 36 and 38are tight buffered optical fibers, the tight buffer layer 402 definesthe outer fiber radius shown as R3. In embodiments, where sensing fibers36 and 38 are not surrounded by tight buffer layer 402, R2 is measuredto the outer surface of coating layer 52 shown for example in FIG. 2. Inaddition, the end sections of cable jacket 200 (adjacent to andsurrounding sensing fibers 36 and 38) are defined by a radius shown asR2.

As discussed above regarding FIGS. 17A and 17B, R2 may be selected toimprove the proportion of vibrational energy directed toward sensingfibers 36 and 38 via refraction. Specifically, Applicant has determinedthat by shaping cable jacket 200 such that R2 is less than 8 times R3,vibrational energy transfer to sensing fibers 36 and 38 can beincreased. In specific embodiments, R2 is less than eight times R3. Ineven more specific embodiments, R3 is 450 microns and R2 is less than3.6 mm and more specifically is less than or equal to 1.8 mm. In anotherspecific embodiment, R3 is 125 microns and R2 is less than 1 mm and morespecifically is less than or equal to 0.5 mm.

In various embodiments, a method of detecting vibrations in anenvironment utilizing a vibration sensing optical cable is provided. Invarious embodiments, the method discussed herein may utilize anycombination of the acoustic sensing enhancement designs of any of thecable embodiments discussed herein. In specific embodiments, the methodincludes the step of placing a vibration sensing cable in theenvironment (e.g., ground 12). In such embodiments, the vibrationsensing cable includes a cable jacket defining an outer surface of thevibration sensing cable, and a vibration sensing optical fiber embeddedwithin the cable jacket. The cable is positioned such that the outersurface of the vibration sensing cable is in contact with theenvironment forming an interface between the outer surface and theenvironment. The cable utilized in this method may be any of the cableembodiments discussed herein.

The method includes transmitting vibrational waves within theenvironment into the cable jacket through the interface. As will beunderstood, the vibrational waves have a direction of travel within theenvironment. The method includes altering the direction of thevibrational waves from the direction of travel within the environment toa path of travel within the cable jacket that intersects thevibrational-sensing optical fiber.

In one embodiment, the step of altering the direction of the vibrationalwaves is accomplished with an acoustic reflector (such as the reflectorsof cables 300, 320 and/340 discussed above) positioned within the cablejacket that reflects vibrational waves traveling within the cable jackettoward the vibration sensing optical fiber. In another embodiment, thestep of altering the direction of the vibrational waves is accomplishedwith a concave surface (e.g., the concave surface of cable 380 discussedabove) located along the outer surface of the cable jacket. The concavesurface is positioned relative to the vibrational waves such thatrefraction of the vibrational waves incident at the concave surfacedirects the vibrational waves traveling within the cable jacket towardthe vibration sensing optical fiber.

In yet other embodiments, the direction of vibrational wave travel isaltered utilizing refraction by selecting the radius of curvature of thecable jacket based on the size and positioning of the sensing fiber. Inone such embodiment, the cable used in the method is cable 400 asdiscussed above. In such embodiments, the direction of vibrational wavesis altered through refraction by forming the cable jacket such that R2is less than 8 times R3. In a specific embodiment, the direction ofvibrational waves is altered through refraction by forming the cablejacket such that R2 is less than 3.6 mm when R3 is 450 microns and morespecifically R2 is less than or equal to 1.8 mm when R3 is 450 microns.In a specific embodiment, the direction of vibrational waves is alteredthrough refraction by forming the cable jacket such that R2 is less than1 mm when R3 is 125 microns and more specifically R2 is less than orequal to 0.5 mm when R3 is 125 microns.

The optical fibers discussed herein include optical fibers that may beflexible, transparent optical fibers made of glass or plastic. Thefibers may function as a waveguide to transmit light between the twoends of the optical fiber. Optical fibers may include a transparent coresurrounded by a transparent cladding material with a lower index ofrefraction. Light may be kept in the core by total internal reflection.Glass optical fibers may comprise silica, but some other materials suchas fluorozirconate, fluoroaluminate, and chalcogenide glasses, as wellas crystalline materials, such as sapphire, may be used. The light maybe guided down the core of the optical fibers by an optical claddingwith a lower refractive index that traps light in the core through totalinternal reflection. The cladding may be coated by a buffer and/oranother coating(s) that protects it from moisture and/or physicaldamage. These coatings may be UV-cured urethane acrylate compositematerials applied to the outside of the optical fiber during the drawingprocess. The coatings may protect the strands of glass fiber. Theoptical transmission elements discussed herein can include a widevariety of optical fibers including multi-mode fibers, single modefibers, bend insensitive/resistant fibers, etc. In other embodiments,the optical cables discussed herein may include multi-core opticalfibers, and in this embodiment, each optical transmission element may bea single, integral optical structure having multiple opticaltransmission elements (e.g., multiple optical cores surrounded bycladding).

In accordance with yet other aspects of the present disclosure,vibration sensing cables may include a cable jacket defining an outersurface having specific contoured patterns formed or provided on theouter surfaces of the cable to create an interference fit with theground environment when the cable is installed into the groundenvironment. For example, the contoured pattern may include a pattern ofridges and valleys mechanically formed into the outer jacket whereinaspects of the ground environment may fill in or, for example, in thecase of concrete or asphalt, flow into the contoured pattern to createincreased friction when the ground environment settles or the concreteor asphalt cures. The increased friction in combination with aspects ofa pliable polyethylene jacket and the strength of the strain sensingcables disclosed herein can assist in the survivability rate of cableswhen cracks occur in the ground environment, such as in a concretebridge section or roadway. In many cases, the displacement that resultsfrom crack formation occurs instantaneously. The contoured patterns mayenable the cable jacket to provide a degree of yield and absorb theinfinite forces created by instantaneous displacement from zero to crackwidth so that the optical fibers and cable survive.

Unless otherwise expressly stated, it is in no way intended that anymethod set forth herein be construed as requiring that its steps beperformed in a specific order. Accordingly, where a method claim doesnot actually recite an order to be followed by its steps or it is nototherwise specifically stated in the claims or descriptions that thesteps are to be limited to a specific order, it is in no way intendedthat any particular order be inferred. In addition, as used herein, thearticle “a” is intended to include one or more than one component orelement, and is not intended to be construed as meaning only one.

It will be apparent to those skilled in the art that variousmodifications and variations can be made without departing from thespirit or scope of the disclosed embodiments. Since modifications,combinations, sub-combinations and variations of the disclosedembodiments incorporating the spirit and substance of the embodimentsmay occur to persons skilled in the art, the disclosed embodimentsshould be construed to include everything within the scope of theappended claims and their equivalents.

What is claimed is:
 1. A strain sensing cable for detecting vibrationsin an environment comprising: a cable jacket; a first optical fiberembedded within the cable jacket; a second optical fiber embedded withinthe cable jacket; a tensile strength element embedded in the cablejacket; wherein, when viewed in cross-section taken perpendicular to alongitudinal axis of the cable jacket, the cable jacket defines a firstaxis and a second axis; wherein the first axis intersects the firstoptical fiber, the second optical fiber and the tensile strength elementand the tensile strength element is located between the first opticalfiber and the second optical fiber along the first axis; wherein thesecond axis is perpendicular to the first axis and is located at themidpoint between the first and second optical fibers; wherein the firstand second optical fibers each have a length within plus or minus 0.5%of a length of the tensile strength element such that both the first andsecond optical fibers experience strain caused by vibrations transmittedinto the cable jacket from the environment.
 2. The strain sensing cableof claim 1, wherein an outer dimension of the cable jacket along thefirst axis is greater than an outer dimension of the cable jacket takenalong the second axis, wherein the first optical fiber is locatedadjacent to an outer surface of the cable jacket such that a minimumdistance between the first optical fiber and the outer surface of thecable jacket is less than or equal to 0.5 mm, wherein the second opticalfiber is located adjacent to the outer surface of the cable jacket suchthat a minimum distance between the second optical fiber and the outersurface of the cable jacket is less than or equal to 0.5 mm.
 3. Thestrain sensing cable of claim 2, wherein the cable jacket has an oblongshape when viewed in cross-section taken perpendicular to thelongitudinal axis of the cable jacket such that the outer dimension ofthe cable jacket measured along the first axis is at least twice theouter dimension of the cable jacket measured along the second axis. 4.The strain sensing cable of claim 1, wherein the cable jacket defines anoutermost cable surface surrounding the first optical fiber, the secondoptical fiber and the tensile strength element, wherein the cable jacketcomprises a polymer material that is in contact with outer surfaces ofthe first optical fiber, the second optical fiber and the tensilestrength element.
 5. The strain sensing cable of claim 1, wherein thetensile strength element is a first tensile strength element and furthercomprising a second tensile strength element embedded in the cablejacket, wherein the first tensile strength element and the secondtensile strength element are located on opposite sides of the secondaxis from each other, wherein the first axis intersects the secondtensile strength element, wherein the second tensile strength element islocated between the first optical fiber and the second optical fiberalong the first axis.
 6. The strain sensing cable of claim 5, whereinthe first axis is a horizontal axis that intersects the center points ofthe first optical fiber, the second optical fiber, the first tensilestrength element and the second tensile strength element.
 7. The strainsensing cable of claim 1, wherein, when viewed in cross-section takenperpendicular to a longitudinal axis of the cable jacket, the firstoptical fiber has an unobstructed field of view to an outer surface ofthe cable jacket that has an uninterrupted arc angle greater than 180degrees, and the second optical fiber has an unobstructed field of viewto the outer surface of the cable jacket that has an uninterrupted arcangle greater than 180 degrees.
 8. The strain sensing cable of claim 7,wherein the unobstructed field of view of the first optical fiber isbisected by the first axis and faces in a first direction away from thesecond axis, wherein the unobstructed field of view of the secondoptical fiber is bisected by the first axis and faces in a seconddirection, opposite the first direction, away from the second axis. 9.The strain sensing cable of claim 8, wherein the cable jacket furthercomprises: a first rib protruding outward away from the second axis in adirection along the first axis, wherein the first optical fiber isembedded within the first rib; and a second rib protruding outward awayfrom the second axis in a direction along the first axis opposite fromthe first rib, wherein the second optical fiber is embedded within thesecond rib; wherein the unobstructed field of view of the first opticalfiber has an uninterrupted arc angle greater than 270 degrees; whereinthe unobstructed field of view of the second optical fiber has anuninterrupted arc angle greater than 270 degrees.
 10. The strain sensingcable of claim 1, further comprising a third optical fiber locatedwithin the cable jacket at a position along the first axis between thefirst optical fiber and the second optical fiber.
 11. The strain sensingcable of claim 10, wherein the third optical fiber has a length that isat least 0.1% greater than the length of the tensile strength elementsuch that the third optical fiber is isolated from strain caused byvibrations experienced by the cable.
 12. A strain sensing cablecomprising: a jacket defining an outer surface; a first optical fiberembedded within the jacket such that the first optical fiber experiencesstrain applied to the cable; a second optical fiber embedded within thejacket such that the second optical fiber experiences strain applied tothe cable; and a tensile strength element embedded in the jacket;wherein the tensile strength element is located between the firstoptical fiber and the second optical fiber along a first axis of thejacket; wherein, when viewed in cross-section taken perpendicular to alongitudinal axis of the jacket, the first optical fiber has anunobstructed field of view to the outer surface of the jacket that hasan uninterrupted arc angle greater than 180 degrees, and the secondoptical fiber has an unobstructed field of view to the outer surface ofthe jacket that has an uninterrupted arc angle greater than 180 degrees.13. The strain sensing cable of claim 12, wherein the outer surface ofthe jacket surrounds the first optical fiber, the second optical fiberand the tensile strength element, wherein the jacket comprises a polymermaterial that is in contact with outer surfaces of the first opticalfiber, the second optical fiber and the tensile strength element. 14.The strain sensing cable of claim 13, further comprising a third opticalfiber located within a tube that is embedded in the jacket, wherein thefirst and second optical fibers each have lengths within plus or minus0.5% of a length of the tensile strength element and the third opticalfiber has a length that is at least 0.5% greater than the length of thetensile strength element such that the third optical fiber is isolatedfrom strain applied to the cable.
 15. The strain sensing cable of claim14, wherein the first axis intersects the first, second and thirdoptical fibers, and the third optical fiber is located between the firstand second optical fibers along the first axis.
 16. The strain sensingcable of claim 15, wherein the jacket has an oblong shape when viewed incross-section taken perpendicular to a longitudinal axis of the jacketsuch that an outer dimension measured along the first axis is at leasttwice an outer dimension measured along a second axis perpendicular tothe first axis intersecting the midpoint of the first axis.
 17. A strainsensing cable comprising: a jacket defining a central horizontal axis, acentral vertical axis and an outermost surface of the cable; a firsttensile strength element embedded in the jacket located on a first sideof the vertical axis; a second tensile strength element embedded in thejacket located on a second side of the vertical axis opposite the firstside; a first optical fiber embedded within the jacket and located onthe first side of the vertical axis such that the first tensile strengthelement is located between the first optical fiber and the verticalaxis; and a second optical fiber embedded within the jacket and locatedon the second side of the vertical axis such that the second tensilestrength element is located between the second optical fiber and thevertical axis.
 18. The strain sensing cable of claim 17, wherein thehorizontal axis intersects the first optical fiber, the second opticalfiber, the first tensile strength element and the second tensilestrength element.
 19. The strain sensing cable of claim 18, wherein thejacket has an oblong shape when viewed in cross-section takenperpendicular to a longitudinal axis of the jacket such that an outerdimension measured along the horizontal axis is at least twice an outerdimension measured along the vertical axis.
 20. The strain sensing cableof claim 19, wherein the first optical fiber is located adjacent to anouter surface of the jacket such that a distance between the firstoptical fiber and the outer surface of the jacket along the horizontalaxis is less than or equal to 0.5 mm, wherein the second optical fiberis located adjacent to the outer surface of the jacket such that adistance between the second optical fiber and the outer surface of thejacket along the horizontal axis is less than or equal to 0.5 mm.